| Terminology |
Symbol |
Definition |
| |
|
|
| Wavelength |
 |
Stated in microns (micrometers, 10-6 metre). |
| Wavenumber |
 |
Inverse of wavelength, usually specified in cm-1. |
| Centre Wavelength |
CWL |
Wavelength at centre of passband measured at 50% of peak transmittance. |
| Cut On / Cut Off Wavelength |
 |
Wavelength at which filter begins/ceases to transmit. Usually specified at 5% or 50% absolute transmittance points. |
| Transmittance |
T |
Ratio of transmitted energy to incident energy expressed in percent. |
| Peak Transmittance |
 |
Highest transmittance (occurs at peak wavelength). |
| Bandwidth |
FWHM, BW, HBW, HW |
Width of the passband measured at half the peak transmittance. Expressed in microns or percentage of centre wavelength. |
| Cut On / Off Slope |
|
A measure or the steepness of the transmittance curve
Where lambda 80 and lambda 5 correspond to 80% and 5% to absolute transmittance points. |
| Blocking or Attenuation |
|
Ability to prevent energy at wavelengths outside the passband being transmitted. Expressed as a percentage of incident energy. Typically less than 0.1% (optical density 3.0). |
| Reflection |
R |
Ratio of reflected energy to incident energy expressed as a percentage. |
| Absorptance |
A |
Ratio of absorbed energy to incident energy expressed as a percentage. |
| Effective Index |
 |
The apparent index of refraction of a substrate plus coating stack. |
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Glossary
